changefeedccl: allow users to delete metric labels

We currently allow users to specify a metric label when creating a changefeed to monitor their changefeeds individually or in groups. We also provide a hard limit (1024) on how many metric labels we support. However, there is currently no way for the user to delete a metric label that they are no longer using. This is undesirable for some customers, since leaving a metric label in place can confuse monitoring.

There are some considerations. Since multiple changefeeds can use the same metric label, and also a user may continue monitoring or using a metric label even if there are no changefeeds currently running using that metric label, deleting a metric label would require user input. Also, as discussed in [this thread](https://cockroachlabs.slack.com/archives/C063CP41TG9/p1709764767212419), if a user deletes a metric label we would need to roll up the child scope so the data isn't lost.

Jira issue: CRDB-36436